Can't say for certain since the entire crab isn't visible, but I'd guess that it's a gorilla crab. Pretty common on TBS rock. Keep an eye on the tank and when you see him on an accessible rock, snatch it out and remove the crab. The good thing about these guys is that if he's on a rock and you start to pull it out, they don't jump off. Their natural instinct seems to be to latch on, and maybe dive into a hole. If the latter, squirt a little ro/di water or carbonated water in the hole to flush him out.